THE MOVEMENTS AND REACTIONS OF AMCEBA. 229 



not, like the reflexes, follow unchangeably upon a definite stimulus, 

 but which are modified in their course by new, intercurrent stimuli." 

 In this sense Amoeba, of course, shows automatic behavior. Its 

 responses are by no means unchangeably fixed ; on the contrary, its 

 behavior is often modified by the slightest change in the stimulus to 

 which it is reacting. For examples of this see the chase of one Amoeba 

 by another (p. 200), the following of a rolling ball of food (p. 196). the 

 account of the driving of Amoeba (p. 185), and the description of the 

 method by which Amoeba avoids an obstacle (p. 186). 



Whether these actions agree with the accepted idea of an automatic 

 action in being unconscious we have, of course, no means of knowing. 



VARIABII-ITY AND MODIFIABILITY OF REACTIONS. 



There is little that can be said on this point. Verworn (1890, a, p. 

 271) says that when an electric current is passed through a prepara- 

 tion containing many Amcebse, some respond strongly, while others 

 do not; thus different individuals vary in their responsiveness. Fur- 

 ther, a given individual may become accustomed to the current, at first 

 responding to it, later not responding. Doubtless such phenomena of 

 acclimation are common in the reactions to all sorts of stimuli. 



Rhumbler (1898, p. 203) shows that when Amoebae are engaged in 

 taking Oscillaria filaments as food, light thrown upon them modifies 

 them physiologically in such a way that they eject the food. The 

 nature of the reaction is thus shown to depend partly on the physio- 

 logical condition of the animal. 



There is no direct experimental evidence as yet. so far as I am aware, 

 that Amoeba shows memory.* Experimental evidence as to whether 

 the reactions of a given Amoeba to a given stimulus are modified bv 

 previous stimuli received is very difficult to obtain, principally because 

 it is practically impossible to make succeeding stimuli alike, so that 

 one cannot tell whether a difference in the reaction is due to a differ- 

 ence in the present stimulus or not. Possibly there is a faint indication 

 of something akin to memory shown in the facts described on page 201. 

 Here a smaller Amoeba which had been ingested as prey escaped from 

 the posterior end of the captor ; the latter thereupon reversed its move- 

 ments, came up with the escaping prey, and again ingested it. In the 

 interval between the complete separation of the prey from its captor 

 and its recapture, the behavior of the captor would seem to have been 

 determined by some trace left within it by the former possession of the 



*The word memory is, of course, used here of the objective phenomenon that 

 in many animals present behavior is modified in accordance with past stimuli 

 received, or past reactions given. Of possible subjective accompaniments of this 

 objective phenomenon \ve, of course, know nothing directly so far as the lower 

 organisms are concerned. 



